Duplex stainless steels are excellent in corrosion resistance and weldability, and in particular excellent in sea-water corrosion resistance and strength compared to ferrite stainless steels or austenitic stainless steels. Accordingly, duplex stainless steels have been widely used for a long time as industrial materials for their easiness of reducing thickness and economic efficiency. Particularly, high Cr-high Mo duplex stainless steels are excellent in corrosion resistance and strength, and thus are used in various fields such as line pipes, components of heat exchangers, process steel tubes and pipes for oil and chemical industrial usage, and oil country tubular goods. Currently, because of increasing deeper sea oil wells and reduction in thickness of materials, materials having further higher strength are required in umbilical tubes for oil wells and others. Greater contents of Cr and Mo in duplex stainless steels, however, more likely cause precipitation of intermetallic compounds (σ phase, χ phase) that are hard and brittle in a temperature range of about 800 to 1000° C. This is because of the following reasons.
A solid billet of a duplex stainless steel is produced in such a manner that an ingot is hot-forged or hot-rolled into a longer-length cast piece, and the cast piece is allowed to cool, and thereafter the cast piece is subjected to machining such as cutting-off and cutting. In a high Cr-high Mo duplex stainless steel, a σ phase is likely to precipitate, in particular during air cooling, which significantly hardens a material thereof, and is likely to cause cracks, so that it becomes difficult to cut off or cut the material in various machining. Hence, it is preferable to suppress precipitation of the σ phase to be as small as possible in production of the duplex stainless steel, and various suggestions have been made, such as reduction in the contents of Cr and Mo, and modification of heat treatment conditions and cooling conditions.
For example, Patent Document 1 suggests a duplex stainless steel in which a phase stability index PSI (=3Si+Cr+3.3Mo) is defined to be 40 or less. Patent Document 1 describes that no σ phase or the like is formed under heating, heat treatment and welding conditions during normal hot working of this duplex stainless steel.
Patent Document 2 suggests a producing method of a duplex stainless steel that heats a duplex stainless steel at a temperature of 1110° C. or more, and subjects this steel to hot working into a seamless steel tube, in which the steel is reheated so as to have a temperature within a range satisfying 800+5Cr+25Mo+15W≦T(° C.)≦1150 after finishing rolling, and thereafter rapidly cools the steel. Patent Document 2 describes that it is possible to produce a high-strength duplex stainless steel tube having no precipitation of the σ phase, and excellent in corrosion resistance.
Patent Document 3 suggests a duplex stainless steel having ferrite amount and a PRE value that are adjusted to be within a predetermined range. Patent Document 3 describes that it is possible to produce a duplex stainless steel excellent in sea-water resistance through this configuration. Patent Document 4 suggests a duplex stainless steel whose Mo content is reduced so as to suppress formation of the σ phase, and having ferrite amount and a PREW value that are adjusted to be within respective predetermined ranges. Patent Document 4 describes that it is possible to produce a duplex stainless steel excellent in warm workability, crevice corrosion resistance, and structural stability through this configuration.
Patent Documents 5 and 6 suggest duplex stainless steels having ferrite amount, respective PREW values of an austenite phase and a ferrite phase, and a ratio therebetween that are adjusted to be within respective predetermined ranges. Both Patent Documents 5 and 6 describe that it is possible to produce a duplex stainless steel excellent in corrosion resistance and structural stability.